A finite element model for single track laser cladding Al2O3 ceramic on Ti6Al4V substrate was established using ANSYS software. By observing the molten pool's morphology and measuring the temperature, the simulation shows a good agreement with the experiment, which indicates that the model is reliable. Furthermore, the variation of temperature gradient during the cooling stage and the effect of preheating on temperature gradient were both analyzed. The results show that the temperature gradient decreases with the temperature decreasing during the cooling stage. When the temperature drops to the plasticity point of alumina, the edge of the bonding between layer and substrate exhibits a higher temperature gradient. So it is easy to form cracks at the edge of the bonding. And preheating substrate could reduce the temperature gradient sharply. The higher the preheating temperature is, the lower the temperature gradient is. So it is possible to restrain the crack formation by preheating substrate.